Interferometry is used in the testing of semiconductor wafers to provide measures of flatness and thickness variations. Generally, the opposite side surfaces of the wafers are both measured for flatness, and the two flatness measures are compared to determine variations in thickness. Of the two, the measurement of thickness variations is of most importance because the semiconductor wafers, which have a very high aspect ratio of diameter to thickness, tend to conform to their mounts.
Semiconductor materials, such as silicon and gallium arsenide, are generally not transmissive within the visible spectrum. However, the opposite side surfaces of some semiconductor wafers are highly polished so that both side surfaces can be measured by reflecting a test beam from each surface and by combining the reflected test beam with a reference beam to produce an interference pattern representative of surface variations.
One available technique using a single interferometer measures one side surface at a time. Between measures, the semiconductor wafer is remounted. Each mounting can cause distortions in the shape of the semiconductor wafer, which reduces accuracy of both flatness measurements as well as the calculated thickness variations. The remounting is also time consuming.
Another available technique measures both side surfaces simultaneously using two interferometers. Although mounting distortions can still affect the flatness measurements, the relative measure of thickness variations is largely independent of the mounting. However, the positions of the two interferometers must be exactly known with respect to the mounting. Also, the two interferometers are expensive and difficult to maintain in the exactly known positions.
U.S. Pat. No. 4,653,922 to Jarisch et al. discloses an interferometer that includes an arrangement of reflective optics for traversing both side surfaces of "non-transparent" wafers with a single test beam. Any variation in the optical path length of the test beam with respect to a reference beam is interpreted as a variation in thickness. One embodiment combines two flat mirrors with a diffraction grating for guiding the test beam, and another embodiment substitutes a folding mirror for the grating. All of these reflective optics are quite large and cumbersome to position in required alignment.
These and other problems with known interferometric techniques for measuring semiconductor wafers are made worse by the increasing size of these wafers, which now measure as much as 30 cm in diameter. One embodiment of Jarisch et al.'s interferometer requires mirrors two to three times the wafer diameter, which is impractical for such large wafers.